TESTING MODELS OF LOW-δ18O SILICIC MAGMATISM IN THE MID-MIOCENE SANTA ROSA-CALICO VOLCANIC FIELD, NV

Low-δ¹⁸O silicic magmas are found in many volcanic provinces throughout the world, including the Snake River Plain-Yellowstone volcanic province (SRPY). The origin of SRPY low-δ¹⁸O silicic magmas is controversial, and centers on two disputed models: [1] a caldera collapse model that proposes a reworking of the hydrothermally altered intra-caldera fill into the underlying silicic magma body, where each successive eruption lowers the δ¹⁸O of the magma eventually producing a low-δ¹⁸O magma and [2] melting of previously hydrothermally altered mid-upper crust in a non-caldera setting, to form low-δ¹⁸O magmas. The mid-Miocene Santa Rosa-Calico volcanic field (SC) lies in northern Nevada. Brueseke and Hart (2008) described the geology and petrology of the SC, but did not deal with the ¹⁸O compositions of any locally sourced silicic magma. In the existing geological framework of the SC, this project aims to evaluate the two disputed models for low-δ¹⁸O silicic magma generation by analyzing the δ¹⁸O values of SC silicic eruptive products. Fifteen representative samples of locally erupted silicic units (e.g. ash-flow tuffs and lava flows)were chosen for ¹⁸O analyses based on Sr-Nd-Pb isotope compositions, whole rock geochemistry, and field/temporal relationships. Each sample was crushed, sieved, and quartz and feldspar crystals were handpicked, described, and analyzed for their ¹⁸O compositions. Our results show that low-δ¹⁸O values exist in the SC and are limited to the youngest erupted silicic unit, the 15.7 to 15.4 Ma Cold Springs tuff, which was also the only unit erupted from a caldera. Cold Springs tuff δ¹⁸O values range from 2.36 to 4.05‰ for feldspars that are in equilibrium with quartz. Older lava flows that are not petrogenetically related to the Cold Springs tuff (or in some cases, to each other), are characterized by normal δ¹⁸O feldspar values (in equilibrium with quartz) that range from 7.19 to 10.04‰. Data synthesis is underway, but our current hypothesis is that previously altered crust was melted to produce the low-δ¹⁸O Cold Springs tuff magmas based primarily on their Sr-Nd-Pb isotopic compositions and previous work performed on broadly coeval, local hydrothermal systems (e.g. Buckskin-National).